The present invention relates to a control for reading data recorded on a magnetic disk and for writing data on the magnetic disk in a magnetic disk apparatus. More specifically, the present invention is directed to a magnetic disk control apparatus for reading and writing data in response to a read/write control signal of data.
There is a magnetic disk (simply referred to as a "disk") known as "embedded servo", wherein control signal regions in which servo information to designate which head of is outputting actual data has been recorded, and data regions in which the read data are recorded are alternately connected with each other to constitute a recording track. Such an embedded servo type disk has a merit with respect to data recording density, as compared with the conventional magnetic disk apparatus in which the tracks on which the real data are recorded and the tracks on which the servo signals for controlling the position of the magnetic head are recorded are formed separately on the magnetic disk.
In the conventional embedded servo type magnetic disk apparatus, the read/write control signal, namely the index sector pulse (ISP) signal is produced in response to the servo information from the control signal of the track, and the read data are transferred to an external unit, for instance, a host computer in response to the generation of the ISP signal (concerning the "data transfer" operation).
In FIG. 1, there are shown an arrangement of a signal for a track 1 in the conventional embedded servo type magnetic disk apparatus, and also a timing chart of generating the ISP signals corresponding thereto.
In the conventional magnetic disk apparatus employing the embedded servo, only one index sector pulse signal functioning as the base point for the read/write control is outputted in response to the servo information which has been read out from the servo (control signal) region of the recorded disk plane. That is to say, in response to signal edges of the servo information Sn, S.sub.(n+1) and S.sub.(n+2) read out from the track 1, the index sector pulse signals ISP.sub.n, ISP.sub.(n+1), and ISP.sub.(n+2) are formed. In response to these ISP signals, the read/write controls for the corresponding data ID.sub.n, ID.sub.(n+1) and ID.sub.(n+2) are performed. In other words, a single read/write operation is carried out with respect to a single servo region. The above-described prior art is opened described in, for example, Japanese patent application laid-open No. JP-A-1-194170 filed on Jan. 29, 1988 by Toshiba Corporation.
In data plane servo type magnetic disk apparatuses, a data region on which read data is recorded is referred to as a physical data length. The data length capable of being recorded on a single data region sandwiched by control signal regions for control signals, on which servo information to designate heads of data has been recorded, is predetermined by the types of magnetic disk apparatuse. This data length is fixed and thus cannot be arbitrarily varied by users.
In case that a data transfer demand is made from an external apparatus, e.g., a host apparatus to the magnetic disk apparatus, the data length accessed by the host apparatus within one access operation, namely the logic data length is not always coincident with the physical data length. Since the physical data length is fixed, this physical data length cannot be coincident with the logic data length.
When the logic data length is shorter than the physical data length, a data region corresponding to a difference between these two data lengths is not effectively utilized. This implies that a practical storage capacity of a disk is lowered and high recording density is deteriorated.
Furthermore, in the conventional embedded servo type magnetic disk apparatus, if the servo information cannot be read due to damages or the like of the disk, the ISP signal cannot be produced, so that the data stored in the data region positioned just after this damaged servo information cannot be read out from the disk.